The term lithography denotes, in semiconductor technology, a method for transferring circuit patterns of microelectronic components and integrated circuits onto a silicon semiconductor slice, the wafer. For this purpose, firstly a mask is produced which contains the pattern in the form of differences in transparency for the beams which are used to transfer said pattern onto the wafer. The wafer surface is coated with a radiation-sensitive photoresist and exposed through the mask. Semiconductor structures are transferred onto the photoresist, by means of a so-called lithography scanner. During the subsequent development, depending on whether a positive or negative resist is involved, the exposed or unexposed photoresist is dissolved away and the wafer surface is uncovered at these locations.
On account of the decreasing feature size of semiconductors, the fabrication of modern semiconductor elements, such as, for example, memory chips and CPUs, requires a resolution which makes it necessary to use extremely short-wave radiation of approximately 13 nm with a quantum energy of approximately 92 eV (EUV radiation). The irradiation wavelengths of 248 nm (UV radiation), 193 nm (DUV radiation) or 157 nm (VUV radiation) used heretofore no longer suffice to produce the shrinking structures. As the feature size and wavelength decrease, however, there is an increase in the requirements made of the resists used, the so-called resist material, as far as both the sensitivity and the line roughness are concerned.
The changed requirements made of resists require the test systems thereof to be adapted, said test systems being used to determine the resist properties with varying irradiation before series production of the wafers.
EUV radiation is absorbed by matter to an extremely high degree. It is necessary, therefore, for the EUV radiation to be guided under ultra-high vacuum conditions. The source of the EUV radiation is a thermally emitting plasma. In contrast to the lasers used heretofore, plasma emits in a very broad band, so that DUV, VUV and UV radiation are also obtained besides the desired EUV radiation. It is necessary, therefore, to keep this radiation away from the resists by means of spectral filters.
So-called EUV beam tubes on synchrotron storage rings which emit monochromatized EUV radiation constitute a highly stable EUV radiation source for researching EUV lithography technology. Such EUV radiation sources emit very short radiation pulses (<1 ns) with repetition frequencies of a few MHz, so that these EUV sources are often referred to as quasi-cw sources. On EUV beam tubes on synchrotron storage rings, for the purpose of testing resists applied on slabs, individual fields have been irradiated sequentially with different radiation doses in order to determine the influence of the radiation dose on the resist. Moreover, on synchrotron storage rings, a plurality of resist-coated fields have also already been exposed simultaneously, a rapidly rotating diaphragm wheel arranged upstream of the resist layer in the beam path performing the function of a neutral wedge filter. The diaphragm apertures arranged radially on the wheel have different sizes, so that the individual fields are exposed to the radiation for different lengths of time during each revolution. Reproducible radiation conditions on the individual fields of the object are only possible with the diaphragm wheel because the EUV radiation source exhibits virtually steady-state behavior on account of the high repetition frequency and radiates very stably.
Finally, irradiation experiments on resists have already been carried out using low-power laboratory radiation sources for EUV radiation, in each case only an individual field on the object having been irradiated. EUV laboratory radiation sources generate a dense and hot (>200 000° C.) plasma and emit the EUV radiation exclusively in very short pulses (typically 100 ns) with very low repetition rates (typically 10-1000 Hz).